timer-info

Arguments:
timer

This generic function provides a place for an application to store
arbitrary information about its current use of a timer. Typically,
when an application starts a timer, it will store an object here to
which the timer's on-timer function may (or may not)
apply its functionality when it is invoked.

The value of this property, which can be set with setf and this function, can be any Lisp object.

(The timer
class can of course be subclassed in order to add custom slots for
this or any other purpose, but this single timer-info slot is provided
since such a handy place is often needed due to the asynchronous
nature of timers, and a single slot is often sufficient.)